Using a DE is pretty straightforward but you might be out of luck if you use something like dwm.

Find correct DPI of your display currently used by X server

xdpyinfo | grep -B2 resolution

This si probably the correct value. If not, you can calculate it by converting your screen size to inches and dividing the resolution by display length.

xrandr | grep -w connected
Output of the last command

The output on my machine

The above block spits your screen resolution and physical size. Now divide it by display length in inches.

Create/modify the file ~/.Xresources and append the following line (replace 96 with your DPI)

Xft.dpi: 96

and have it processed by the startup file (like .xinitrc)

xrdb -merge ~/.Xresources